A battery charger is a device used to put energy into a rechargeable cell or battery by forcing an electric current through the cell or battery. The charge current generated by the battery charger is provided responsive to a provided input voltage which in some applications may be provided from an AC adapter or USB port. Battery chargers accept the input voltage from the external source, such as an AC adapter or USB port, and using a chemistry dependent charging algorithm, charge the battery or battery cell to a desired voltage level. A common issue encountered in battery charging applications is that the input voltage may exceed the maximum voltage rating of a charging transistor included within the charging circuitry. This may cause damage to the device that is being charged and potentially lead to battery failures within the electronic device.
One solution which has been used to prevent damage from an over-voltage condition within an electronic device being charged is the insertion of an over-voltage protection circuit between the input voltage and the battery charger. Referring now to FIG. 1, there is illustrated a prior art embodiment of an over-voltage protection circuit 100. An input voltage VIN is applied to node 102 of the over-voltage protection circuit 100 at node 102. The input voltage VIN is applied to a positive input of an over-voltage comparator 104. A reference voltage VREF is also applied to the over-voltage protection circuit 100 at the negative input of the over-voltage comparator 104. The input voltage VIN is applied through a transistor 106 having its source/drain path connected between node 102 and node 108. The gate of transistor 106 is connected to the output of the over-voltage comparator 104. The transistor 106 is an over-voltage protection MOSFET which is controlled by the over-voltage protection comparator 104 connected to a gate of the transistor 106. Transistor 106 is rated to a voltage that exceeds the maximum voltage that a system would be provided as the input voltage VIN. When the input voltage VIN applied to the input node 102 is determined by the over-voltage comparator 104 to exceed the reference voltage VREF, a control signal applied to the gate of transistor 106 from the comparator 104 turns off transistor 106 to disconnect the input voltage VIN from charging the battery.
A second transistor 110 has its source/drain path connected between node 108 and the output voltage node VBAT 112. The transistor 110 is a charging MOSFET which is controlled by the chemistry dependent charge control circuit 114. Often a device being charged needs some current from the input voltage directly but cannot tolerate high input voltages. Thus, a connection is made at the drain of the transistor 106 at node 108 to provide an over-voltage protective input voltage shown as VIN—BYPASS thru transistor 110. The transistors 106 and 110 are low impedance transistors which are more expensive than higher impedance transistors and larger. There is a need to provide a smaller, less expensive and simplified method of providing over-voltage protection.